Spoiler warning: Do not read on unless you’ve seen “The Flash” crossover episode of “Supergirl,” which took place during Season 1, Episode 18, titled “Worlds Finest.”

“Supergirl” and “The Flash” just pulled off the first inter-network crossover in recent memory, allowing Kara Zor-El (Melissa Benoist) to win back the trust of National City and make a new friend in Barry Allen (Grant Gustin), before the Scarlet Speedster raced back to his own universe over on The CW. But while Barry has to focus on Zoom, who is plaguing both Earth-1 and Earth-2, Kara will have to contend with the mysterious Myriad, which seems to have turned James Olsen (Mehcad Brooks) and every other human in her city into mindless drones.

Below, “Flash” and “Supergirl” executive producers Greg Berlanti and Andrew Kreisberg break down the crossover’s twists and preview what’s ahead for Kara in the final two episodes of “Supergirl’s” freshman season.

Mind Over Myriad: We now know that Myriad enables the Kryptonians to control the minds of humans, and Kreisberg says that in the next two episodes, “You’re finally going to discover why Astra did it. Originally when we were developing the series… Astra was trying to mine the core of Krypton and now she was [going to be] trying to mine the core of Earth, just to show the synchronicity, but Greg rightly pointed out that it should be something much more emotional and visceral. The more we talked about it, the whole idea of mind control [came up],” he explains. “As you’ll hear in subsequent episodes, you start to get very cogent arguments on the bad guys’ side for why Myriad is the way to go. For us, we always love it when the bad guys have a point – they’re leaving out the part about killing a whole bunch of people or you’re losing your free will, but next week Non gives a speech: ‘what has your free will gotten you? A planet full of reality TV shows and politicians who can’t stand each other, and you’re all [standing] by as global warming is coming to destroy your planet, and what if everybody was working in lockstep, what if everybody was moving in the same direction, what if everybody was working together – wouldn’t that be better?’”

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While we humans obviously enjoy our free will (and some reality TV), Kreisberg points out that “Astra tried to do Myriad on Krypton and she was arrested and thrown in jail, and when she got out of jail, Krypton was destroyed. So as far as Astra’s concerned, she was right: ‘they didn’t listen to me and now they’re all dead.’ So she had proof positive that ‘my plan might’ve saved Krypton and my plan can save Earth.’”

Since Kara’s sister Alex (Chyler Leigh) and mentor Hank Henshaw — aka J’onn J’onzz (David Harewood)– are off on a mission to track down Alex’s father, who is apparently still alive, Kara will need some help tackling Myriad and Non in the final two episodes. “She has a couple of surprising players that you wouldn’t expect to be the two people who are completely unaffected by Myriad to help her,” Kreisberg teases.

Manhunter on a Mission: Kreisberg promises that we’ll catch up with Alex and “Hank” in next week’s episode; “we see them on the run in a very comical way.” Alex will obviously been torn between the need for answers about her father and what’s going on back in National City, the producer says. “She is on a mission, but Myriad taking over the entirety of National City is going to put into question which mission she should be pursuing; she knows her father’s alive and he’s out there, but the entirety of National City has been taken over and there’s going to be some discussion with Hank about which danger is more immediate and pressing.”

Unfortunately for Jeremiah Danvers, he’s apparently being kept under lock and key thanks to a secret government program called Project Cadmus, which DC Comics fans know well. “You’re not gonna see Cadmus this season, it’ll be something that gets saved for Season 2,” Kreisberg says, explaining, “we did run out of runway a little bit. It’s going to remain more of an evil spectery thing that’s out there, but it’s definitely something that we’re going to pursue. Cadmus has such a rich history in the comics and the animated shows. There’s so much there to mine — it’ll definitely be there in Season 2.”

Sealed with a Kiss: Barry was able to offer Kara some sound advice about when to speed things up and when to take them slow, and our heroine took that advice to heart by finally making a move on James — but once again, the timing didn’t quite work out. “Barry’s advice about when to go slowly and when to go fast really resonated with her and I think she realized that they’ve had so many fits and starts throughout the course of the year and so much of it was because one or the other of them didn’t stop talking and just act. The idea that just as they kiss, Myriad takes over… poor Kara’s horrible reaction after finally making the first move and he’s turned into a zombie just felt like the best Whedon-esque pathos,” Kreisberg says of the cruel twist they decided to throw at Kara and James’ first kiss.

Earth-what? Fans of “The Flash” know that Barry already has some experience with traveling to alternate universes, but the producers say they purposefully kept the location of Kara’s Earth vague, mostly to avoid any DC Comics fans who know the specifications of each Earth that’s been introduced into the multiverse canon already. “I know the deeper you get into the Earths, the more specialized they become,” Kreisberg laughs. “This is an unknown number Earth.”

The Devil You Know: While Barry was unable to find any of his “Flash” allies on Kara’s Earth — no Caitlin Snow, Cisco Ramon or Harrison Wells — that doesn’t mean the producers have ruled out the possibility of familiar doppelgangers from other shows on “Supergirl” down the line. “There is room for that, absolutely,” Berlanti says.

“We just wanted to make sure that Barry didn’t have other options other than to work with Kara,” Kreisberg explains wryly. “‘Oh, there’s a Harrison Wells, buh-bye, crossover over!'”

Flash in the Pan: The crossover might be one-sided, but Grant Gustin tells Variety that Barry’s trip to another universe will still have an effect on him, going into “The Flash’s” last batch of episodes.

“My favorite thing about it was we were shooting our episode 18 of ‘Flash’ when I was simultaneously popping back and forth and doing ‘Supergirl’s’ episode 18, and it’s the episode where Barry does disappear and go to this other Earth, so it coincided perfectly,” he says. “Barry’s kind of going through it, as he does towards the end of the season, and it’s heavy and he’s been kind of down and this event — which doesn’t read at all in ‘The Flash’ script as affecting Barry, but because he’s gone and he’s on ‘Supergirl’ — when he comes back, it really ended up affecting my mood on that episode of ‘Flash,’ and kind of changed the arc for Barry for the next couple episodes. It helped give him some confidence knowing he had this other ally out there; he just came back with a more positive outlook than I would’ve expected him to after breaking the dimensional barrier and disappearing for a full day.”

Kreisberg agrees that the crossover has a little less of an impact on “The Flash,” because “this was really meant to be a more one-sided crossover,” but does promise that “you will know precisely when it occurred on ‘Flash,’ but we don’t dwell too much on it.”
Crisis on Infinite Earths? The biggest question for many fans following the crossover may be “when can we do it again?” and the producers are certainly open to another, larger meeting between all the shows in Berlanti’s superhero universe — including “Arrow,” “The Flash,” “Supergirl” and “DC’s Legends of Tomorrow.”

“We would love that,” Berlanti says. “We want to wait and see the audience reaction to something like this, but just as storytellers and fans and fans of the actors, we love it when we can figure out a way, creatively, to do it, and hopefully everybody enjoys it.”

Berlanti admits that they always hoped to do a crossover between “Supergirl” and The CW shows, if the Powers That Be were on board. “From the day we cast Melissa, the notion of her in scenes with Grant — or someday maybe in the best of all worlds with Stephen [Amell, star of ‘Arrow’] as well, it’s exciting to think of them on screen together. There was a tonality to the shows. But a few things definitely had to go right – we were introducing this notion of Earth-2 and the multiverse on ‘Flash,’ and that had to work; we said very openly that ‘Flash’ and ‘Arrow’ exist in a universe where there’s no Superman or you would’ve heard about him – they would’ve at least had one conversation about him – and certainly Supergirl as well. So that was one thing, and there was the notion that it was two separate networks, so it was definitely challenging in that way because it’s not so convenient, it’s two separate cities that they shoot in. So you wanted the show to work and stand on its own on CBS, have its own audience, and we thought we might wait ’til the second season, but after Christmas we came back and we were planning out the back half of the year, and we saw a way we thought we could do it, and it felt like the right time for him to enter her world and help her out, and so it all came together. Everybody was very enthusiastic and supportive.”

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Spoiler warning: Do not read on unless you’ve seen “The Flash” crossover episode of “Supergirl,” which took place during Season 1, Episode 18, titled “Worlds Finest.” “Supergirl” and “The Flash” just pulled off the first inter-network crossover in recent memory, allowing Kara Zor-El (Melissa Benoist) to win back the trust of National City and make […]

Spoiler warning: Do not read on unless you’ve seen “The Flash” crossover episode of “Supergirl,” which took place during Season 1, Episode 18, titled “Worlds Finest.” “Supergirl” and “The Flash” just pulled off the first inter-network crossover in recent memory, allowing Kara Zor-El (Melissa Benoist) to win back the trust of National City and make […]

Spoiler warning: Do not read on unless you’ve seen “The Flash” crossover episode of “Supergirl,” which took place during Season 1, Episode 18, titled “Worlds Finest.” “Supergirl” and “The Flash” just pulled off the first inter-network crossover in recent memory, allowing Kara Zor-El (Melissa Benoist) to win back the trust of National City and make […]

Spoiler warning: Do not read on unless you’ve seen “The Flash” crossover episode of “Supergirl,” which took place during Season 1, Episode 18, titled “Worlds Finest.” “Supergirl” and “The Flash” just pulled off the first inter-network crossover in recent memory, allowing Kara Zor-El (Melissa Benoist) to win back the trust of National City and make […]

Spoiler warning: Do not read on unless you’ve seen “The Flash” crossover episode of “Supergirl,” which took place during Season 1, Episode 18, titled “Worlds Finest.” “Supergirl” and “The Flash” just pulled off the first inter-network crossover in recent memory, allowing Kara Zor-El (Melissa Benoist) to win back the trust of National City and make […]

Spoiler warning: Do not read on unless you’ve seen “The Flash” crossover episode of “Supergirl,” which took place during Season 1, Episode 18, titled “Worlds Finest.” “Supergirl” and “The Flash” just pulled off the first inter-network crossover in recent memory, allowing Kara Zor-El (Melissa Benoist) to win back the trust of National City and make […]

Spoiler warning: Do not read on unless you’ve seen “The Flash” crossover episode of “Supergirl,” which took place during Season 1, Episode 18, titled “Worlds Finest.” “Supergirl” and “The Flash” just pulled off the first inter-network crossover in recent memory, allowing Kara Zor-El (Melissa Benoist) to win back the trust of National City and make […]